1. Field of the Invention
The present invention relates to a method of servicing an inkjet printhead without removing the printhead from a carriage of a printer and in particular to methods of servicing a printhead by the utilisation of a controlled puddle of ink generated on the outside of the nozzle plate of the printhead.
2. Discussion of the Background Art
The present invention relates to the art of inkjet printing mechanisms whether of the thermal or piezo variety which may be included in a variety of different products including copiers and facsimile machines in addition to standalone printers either desktop mounted, portable or freestanding. Herein a freestanding printer will be used to illustrate the present invention. Printers of this type have a printhead carriage which is mounted for reciprocal movement on the printer in a direction orthogonal to the direction of movement of the paper or other medium on which printing is to take place through the printer. The printer carriage of a color printer typically has two or more, usually four, thermal ink jet printheads mounted thereon which may be removable. Each of the printheads contains or is attached to a remote supply of ink which is fed via ink channels within the printhead to an ink ejection mechanism generally in the lower part of the printhead and ejected as drops through a nozzle plate having numerous small orifices or nozzles therethrough. For thermal (as opposed to piezo-electric) inkjet printheads ink channels or conduits lead to firing chambers each associated with heater elements, such as resistors, which are energized to heat ink within the firing chambers. Upon heating, an ink drop is ejected from a nozzle associated with the energized resistor.
To service, that is clean, maintain, protect or recover the correct operation of the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer so the printhead can be moved over to the station for servicing. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and prevents drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit or other mechanism that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as xe2x80x9cspitting,xe2x80x9d with the waste ink being collected in a xe2x80x9cspittoonxe2x80x9d reservoir portion of the service station. After spitting, uncapping, priming or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the face of the printhead.
A factor in the servicing of printheads is that, to improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media. Such new faster drying ink formulations have placed additional demands on the servicing of printheads.
A further factor in the servicing of inkjet printheads is that the lifetimes required of the printheads is increasing, particularly for printheads that are utilised in combination with large volume ink reservoirs which are remote from the printhead (so called xe2x80x9coff-axisxe2x80x9d systems) and which may be replaced without replacing the printhead. Thus, increased levels of, or more effective servicing of printheads are required and furthermore such servicing must cause very little wear or damage to the printhead if it is to have a long lifetime.
A particular problem, that is exacerbated by the use of inkjet printheads for longer periods of time is that the printheads are very sensitive to contamination by either small air or gas bubbles generated during use of the printhead or by solid particles either left within the printhead from manufacturing processes or entering the printhead together with the ink. While this problem has been attempted to be resolved by for example the use of a filter within the printhead as described in EP 0875385, such a filter only addresses the attempted prevention of these problems and does not provide a solution should they occur.
According to a first aspect of the present invention, there is provided a method for servicing an inkjet printhead, the printhead having a body comprising an ink chamber in fluid communication with a plurality of nozzles in a nozzle plate and firing means associated with each nozzle for ejecting ink drops from said nozzles during printing operations, mounted within a carriage of a printer, comprising the steps of generating a controlled predetermined pressure differential across the nozzle plate of the printhead to cause the formation of a controlled puddle of ink on the outside of the nozzle plate, and causing the printer to actuate the firing means so that ink is ejected from at least some of said nozzles into said puddle of ink. The present applicants have discovered that the firing of drops into a puddle of ink on the nozzle plate of a printhead causes turbulence within the ink of the puddle which is effective in recovering the correct operation of defective nozzles.
Servicing in this manner has been found to be more effective than servicing by means of conventional, sequential spitting and priming operations. Furthermore this servicing or recovery technique has been found to give little wear to the printhead.
Advantageously, the firing means are actuated repeatedly to eject ink into said puddle of ink, and preferably the repetition rate of actuation for each nozzle is lower than the repetition rate utilised during normal printing operations. This has been found to further enhance the recovery of malfunctioning nozzle which may be due to the large drop volumes ejected or to the timescale for response of the malfunctioning nozzles to this firing.
In a preferred embodiment the servicing method comprising the further step of, prior to actuating the firing means, determining which nozzles of the printhead are able to correctly eject drops of ink during normal printing operations and then, during said actuation step, only firing the nozzles which are correctly operating. It has been found that firing neighbouring nozzles can alleviated a problem with a malfunctioning nozzle and that firing of a malfunctioning nozzle in some circumstances, for example when an ink conduit is block by a particle, can worsen a problem with the nozzle.
Alternatively, subsequent to determining which nozzles of the printhead are able to correctly eject drops of ink during normal printing operations, only the malfunctioning nozzles are fired. For some causes of malfunction, for example nozzles clogged by plugs of dried or drying ink, this is found to be effective.
Preferably, subsequent to actuation of the firing means the majority of the ink forming said ink puddle is drawn back into the printhead through the nozzles. In addition to reducing the amount of waste ink during servicing, this has been found to be a very effective recovery technique particularly for problems caused by particulate matter. It is believed that the flow out of the printhead due to creation of the ink puddle and firing of the nozzles in combination with the flow back into the printhead of the ink puddle serves to move internal contaminants whether bubbles or particulate matter.
Although the ink puddle may be generated by a controlled decrease in the pressure external to the nozzle plate of the printhead, preferably the puddle is generated by a controlled increase in the internal pressure of the ink chamber of the printhead. Advantageously, the increased internal pressure causes the volume of the ink fired through each nozzle into the ink puddle to be higher than the volume of ink drops fired under normal printing conditions.
In a preferred embodiment, the printhead comprises a variable volume air chamber coupled to the ink chamber and having a vent which is in gaseous communication with ambient atmosphere. The generating step then comprises interfacing a source of gas to the vent of the air chamber of the printhead, and delivering a predetermined controlled volume of gas from the gas source at a pressure above ambient atmospheric pressure to the air chamber so that the air chamber expands within the printhead body causing an increase in the pressure within the ink chamber and thus a controlled flow of ink through the nozzles of the printhead to generate the controlled puddle of ink on the outside of the nozzle plate. This method of generating the ink puddle has been found to be particularly controllable.
According to a second aspect of the present invention, there is provided a method for servicing an inkjet printhead, the printhead having a body comprising an ink chamber in fluid communication with a plurality of nozzles in a nozzle plate and firing means associated with each nozzle for ejecting ink drops from said nozzles during printing operations, mounted within a carriage of a printer, the method comprising the steps of generating a controlled predetermined pressure differential across the nozzle plate of the printhead to cause the formation of a controlled puddle of ink on the outside of the nozzle plate, maintaining said ink puddle on the nozzle plate of the printhead for a predetermined period of time, and reversing said pressure differential so as to draw the majority of the ink forming said ink puddle back into the printhead through the nozzles. The present applicants have discovered that the controlled generation of an ink puddle on the nozzle plate of an inkjet printhead and its subsequent reabsorption into the printhead through the nozzles (even without the nozzles being fired) is effective in servicing the printhead.